1. Field of the Invention
This invention relates to a welding device and more particularly refers to a welding device for sealing at least two superimposed foils of thermoplastic synthetic material, having at least one electrically heated, temperature-controlled sealing roller and a counterpressure organ cooperating with the sealing roller, such as a contact roller or the like, between which is a heated zone where the foils are welded while pressure is being applied.
2. Background of the Invention
In known welding devices of the type mentioned, it is customary to equip the sealing roller with electric heating elements, the temperature of which is kept constant at the optimum value for the welding operation by means of control devices. The constant maintenance of the optimum welding temperature is of decisive importance, particularly in the welding of thin films in obtaining a welded seam of good quality. Temperature variations result either in a poor bond and leaks caused thereby, or in welding-through or burning-up of the foils to be welded, and thus lead to unsatisfactory welding results.
In the known welding devices, the accurate maintenance of the temperature, which is absolutely necessary in the interest of making satisfactory welded seams, can be achieved only by a relatively large expenditure for complicated control devices. These control devices require considerable space and in addition, require one or several temperature sensors or temperature interrogating devices which must be housed directly in the sealing roller because of the needed high accuracy of the temperature control.
A sealing roller thus equipped with heating elements and temperature interrogating devices has, however, of necessity, a large diameter, so that such sealing rollers can only be used in large commercial machines and are unsuitable for use in small, handy equipment.
The relatively large dimensions of the known sealing rollers have the further disadvantage that they have a large mass and as a result, a large thermal inertia, which necessitates long heating-up times with correspondingly long cooling-down times. Thus, the welding devices equipped therewith are ill suited for short-term or only occasional use.
A further aggravating disadvantage in known welding devices is the fact that the temperature interrogation devices are arranged only point-wise at the circumference of their sealing rollers. As a result, the heating system reacts sluggishly in the event of sudden local temperature deviation due perhaps to increased heat conduction or the like, at the effective working surface of the sealing roller. If, for instance, one point on the circumference of the sealing roller has a lower temperature than the rest of the points on the circumference of the sealing roller due to sudden, work-related cooling-off, then an immediate reheating is initiated only if the temperature interrogating device is located exactly at the colder point. However, if the sudden cooling-down occurs at a point which is far removed from the temperature interrogating device and is therefore picked up only with a corresponding delay, then temperature variations come about at the circumference of the sealing roller which can easily result in overheating or excessive cooling-down of parts of the working surface of the sealing roller and therefore lead to an unsatisfactory welding result.